MEMS technology has become increasing popular in the past decade. Many inventions for the manufacturing of high technology products have been made based upon the miniaturization using the MEMS technology.
The microvalve is an important component in various fluidic applications, such as miniaturized chemical analysis systems, micro-pumps, and various precision fluidic dispensing spacecraft applications. MEMS technology has given an opportunity for microvalves to be package onto a fluidic board with integrated fluidic channels to interconnect all the parts. This is similar to a printed circuit board in electronics [1].
Microfluidic applications are widespread, and include applications were the fluid is metered or dispensed in a controlled manner, and applications in which the fluid is used as a medium to transport objects or molecules. If the valve channels are wide enough to allow a finely divided powder to pass through without clogging, then the dispensing of a large number of materials is possible. Fluids in the food industry during manufacturing or at point of use mixing of liquids or powders. Precision dispensing of fluids or powders for manufacturing processes are important to conserve material costs, and to provide accurate mixing of component parts either in the gas phase, for example, in semiconductor processing by chemical vapor deposition, or in the liquid phase for liquid phase growth of films, and for definition of the location of the fluid/gel such as in sealing of component or gasket formation or in the assembly of components from liquid precursors. Cosmetics dispensing can make use of valves for dispensing cleaning solvents, and for painting of surfaces. Other uses include the control of fuel injection in an engine, or a propulsion system for aerospace applications of rocket propulsion systems. Biomedical applications include the controlled release of drugs or liposomes of capsules containing drugs.
There are applications where the fluid is used to move something around a system, commonly know as microfluidics, in which either existing well-know chemical or bioanalysis techniques are scaled down in volume to provide faster, higher throughput analysis on small sample volumes, or new methods of analysis that rely on the combinations of chemicals/biochemicals that can be compared in a highly parallel assay, such as biopanning or combinatorial chemistry.
Other examples include sample injection in a gas chromatographic system, high pressure liquid chromatography, field flow fractionation, protein analysis by affinity chromatography, DNA analysis by hybridization, or an immunoassay on a chip. These can be used for numerous applications in medical diagnostics and patient treatment by providing timely bed-side monitoring, or in applications where rapid screening against drug allergic reactions are required. Understandably, bio-detection of toxins and other hazardous agents is a topic where portable, light weight, automated analysis systems will have widespread use. This includes both civilian and military use. Such applications range from checking food and the shipping of packages that are entering the US, to “sniffing” for agents in a high risk environment, to monitoring a nation's water supply.
Valves can further be used to transfer power in miniature hydraulic system, or in the controlled lubrication of surfaces where surface tension driven forces are influenced by the precision dispensing of a fluid is important.
A major advantage of valve miniaturization is that diffusion processes such as mixing and heating, which are typically required in miniature chemical analyses, equilibrate much faster thereby, drastically decreasing assay times. Another benefit of downscaling is that the use of expensive chemical reagents can be reduced, and that sample volumes on the order of only a few μliters [1, 2] are required for an analysis. To achieve these benefits, the dead volume in all fluidic components should be minimized. This can be achieved by MEMS technology and involve the implementation of microvalves that can be integrated with the fluidics [3]. For a microvalve, the minimization of dead volume can be achieved by adopting silicon micro-machining techniques such as KOH etching [4, 5] or deep reactive ion etching [6]. Moreover, the silicon micro-machining has provided an excellent feasibility to integrate other system components such as micro-filters and sensors within the valves [7].
Microvalve development is closely linked with the research in MEMS actuators. The microvalve is involved in many applications, each requiring different characteristics. Even though millions of dollars have been spent in the research, there has not yet been commercialization of the MEMS microvalve, quite unlike the silicon pressure sensor that has been largely commercialized in many engineering applications, including automobile, fluidic device, and jet propulsion.
The fact is that the MEMS valves developed in the research lab are simply not yet reliable and robust enough for commercial application [8, 9, 10]. The current state of research in the microvalve field is not well established, nor oriented into a particular application. The research is mainly focused in the state of innovation, where proper application of the valve is not the focus area. This is a main reason why the MEMS valve has failed to come to the market.
Another challenge in bringing MEMS device to market is the need for CMOS compatibility. Currently, it is a big demand for the MEMS actuator to be CMOS compatible so that it can be integrated in the fabrication with other electrical circuit(s). This has made MEMS fabrication more difficult, as its maximum fabrication temperature should be less than 400° C.
Miniaturization itself is yet another hurdle in the microvalve art. Unlike pressure sensors that require only a silicon membrane as the main component, the microvalve essentially comprises three main parts: an actuator, a cantilever/membrane, and fluidic connections [10, 11, 12, 13], all of which increase the complexity of the design and fabrication of the microvalve. Current sizes of the MEMS microvalve are relatively large in size, that is, larger than 1 mm2 [1, 2, 6, 8, 14-16].
Research in microvalves is widely developed in the Untied States and in Europe. The conventional microvalve can include a valve seat, a membrane/diaphragm, and an actuator. The developments of valve seat and membrane are typically similar and do not show significant differences. Different types of actuators are known, for example: magnetic [1-5], thermal [6-7], piezoelectric [8-9], electrostatic [10-14], pneumatic [15-18], and Hydrogel [19-20]. Each type of actuators has it own characteristics, advantages, and disadvantages. Electrostatic and piezoelectric actuators generally generate small membrane deflection (<5 μm) [10, 13], while pneumatic, magnetic, thermal, and Hydrogel actuators produces larger deflection (>5 μm) [20].
Each actuation type is briefly reviewed and discussed, including its working principal, advantages, and disadvantages.
Electromagnetic Actuation
This type of microvalve utilizes the force generated from magnetic actuation, which can be from “coil to coil” or “magnet to coil” [4, 5] or the combination of both [3]. The design can be quite complex. Yasuhiko Shinozawa [2] has developed a valve that is fabricated with a combination of a micro-machined coil and a permanent magnet. The overall valve dimensions including the actuator are about 5×5×5 mm3. The valve has a vertical displacement of 0.5 mm, which alleviates known clogging problems.
By implementing the permanent magnet, the valve has developed two stable conditions. Its smallest controllable amount of fluid (water) was 0.7 μl/min. S. Bohm [1] designed a microvalve including two separated parts that were fabricated in two-separated processes. The silicon-valve part, with an overall dimension of 7×7×1 mm3 was made by a sandwich construction of two etched silicon wafers with KOH Etching.
Thermal Actuation
This type of microvalve utilizes the elastic deflection technique caused by heating the membrane. The valve design is simpler than the electromagnetic actuator, as it only requires thermal energy to heat up and deflect the membrane. T. Lisec [6] developed a designed for high crushing pressures of approximately 1 bar, delivering flow rates of 700 ml/min=11,667 μl/s. The switching time of this device is about 15 ms, which is extremely low for thermal actuation.
The thermal principle was chosen because it gave both high forces and large deflections in a simple valve construction. The continuous power consumption was in the range of 1-4 Watts. Carlen [7] developed a slightly different design for the valve, using a paraffin micro-actuator as the active element. The entire structure had a nominal dimension of diameter 600 μm×30 μm was batch-fabricated by surface machining. For gas flow rates, the actuation power ranged from 50 to 150 mW with the leak rate of 500 μsccm.
Piezoelectric Actuation
This type of microvalve utilizes the piezoelectric behavior of the material due to the generated electric field at an applied voltage. For this type of design, the membrane actuator can be made directly from a piezoelectric material, or can be attached by layers of piezoelectric material. The piezoelectric force is typically small, since the membrane's structural design is very important in order to produce large displacement—otherwise clogging may occur.
I. Chakraborty [8] designed a valve to meet the rigorous requirements for space applications, such as micro-propulsion, in situ chemical analysis of other planets, or microbiology. It required a small, yet reliable, silicon valve with extremely low leak rates and long shelf lives. Further, it must survive the perils of space travel, which include unstoppable radiation, monumental shock and vibration forces, as well as extreme variations in temperature.
Roberts [9] developed a piezoelectric microvalve for high frequency (>1 kHz) and high pressure applications (>300 kPa). His design has provided large stroke (20-30 μm) and a low closing time (<1 ms).
Electrostatic Actuation
The electrostatic valve utilizes the electrostatic force generated between two surfaces. The amount of force generated depends upon the gap-distance between the surfaces. The electrostatic force is generally small. A small distance between two surfaces is required in order to provide an adequate amount of force for some fluctuation/displacement. However, one has to be very careful because a small displacement may limit the microvalve performance due to clogging.
Ph. Dubois [4] developed a valve that includes a vertically moving, double-clamped Ta—Si—N membrane, located over a small (10 μm) round orifice, machined by deep reactive ion etching through the silicon substrate. In most applications, Ta—Si—N has been used only as diffusion barrier. Nevertheless, this material has numerous characteristics that can be used in MEMS applications.
Vandelli [13] developed a MEMS microvalve array for fluid flow control that use electrostatic actuation, which was used to control air-flow rates of 150 ml/min for a pressure differential of 10 kPa.
Thermo Pneumatic Actuation
This type of valve utilizes the actuation generated from the thermo-pneumatic force, which comes from an increase of gas pressure due to thermal expansion. This type of actuator requires a lot of power consumption and has a long response in valve closing/opening. However, the membrane deflection is generally large to avoid clogging.
W. K Schomburg [6] developed a thermo pneumatic actuated valve with a composite membrane of silicon rubber and Parylene. The design requirements for the valve membrane include they be small in size, they be impermeable to the working fluid, have a large deflection, and have an adhesive compatible surface for bonding the seat chip.
Parylene C forms an effective vapor barrier, while silicon rubber is very soft and elastic. The Young Modulus of the MRTV1 Silicon rubber is 0.51 MPa and of Parylene C is 4.5 GPa. A composite membrane from these two materials has nice flexibility and impermeability properties. In order to keep the membrane soft, a very thin Parylene layer was used compared to the silicon rubber layer.
Rich [17] developed a more complex thermo pneumatic microvalve, which has a sealed cavity below the diaphragm containing a volatile fluid, the pressure of which can be increased by resistive heating to deflect the diaphragm, thus closing the valve. One of these valves suggest a 2000 torr pressure rise with 50 mW input and a is response time.
Hydrogel Actuation
Lastly, the actuation of the Hydrogel microvalve responds to the changes in the concentration of a specific chemical species in an external liquid environment [19]. Baldi [19] made a valve that incorporated a Hydrogel disc sandwiched between a porous plate and a flexible silicone rubber membrane. The swelling of the Hydrogel that is produced by diffusion of the chemical species through the porous plate results in the deflection of the membrane and closure of the valve intake orifice. Baldi's valve was based on a phenylboronic acid Hydrogel and used to construct a valve that response to the changes in the glucose concentration and pH. However the response time is very slow, with the fastest achieved being 16 minutes using a 70 um thick Hydrogel and a 60 um porous back plate.
Robin H. Liu [20] made a valve based on Hydrogel actuation, which has the closing and opening times on the order of 10 s, and claims to be operable at the differential pressure as large as 50 psi.
Based on this review on different type of actuators for microvalves, Table 1 illustrates a brief summary of the advantages and disadvantages of each actuator.
TABLE 1Type of ActuationAdvantagesDisadvantagesThermalThe amount of deflection can be adjustedPower consumption can be large and not applicablefrom the amount of power input to thefor application where the energy consumption isheating element.limitedIt is simple, basically the elongation ofThe thermal expansion may cause loosening to thematerial due to the thermal expansion givesupport, which may introduce some leakage in anthe actuation to the membrane [6, 7]improper designIt can be integrated with shape memoryThe valve closing time may not be the same as thematerial at limited number of cycles in orderopening timeto give pre-stress on the membrane forincreasing closing force. [11]Using the optimization of the flow back-The heating to fluid may not be applicable to somepressure in closing the valve reduces theMEMS application. For fluid with low evaporationleaking possibility and improved the devicetemperature, may evaporate during the valvecrushing pressure [6]operation.PiezoelectricFlexible, the amount of membraneIt requires high power consumption. Recentdeflection can be adjusted from the amountpiezoelectric valves require continuous power toof potential voltage applied to thekeep the valve in the open/close position.piezoelectric material [8]It is pretty stable in the environmentalIt generally produces a small amount of deflectioncondition that involves high thermalat a given amount of voltage.fluctuation, shock, and vibration. It mayproduce a reliable microvalve suitable forheavy-duty application [9]Typical piezoelectric disk requires highThe structure and fabrication processes may bevoltage to produce substantial deflection, thecomplex, it requires enormous amount of time in tolaminated piezo stacks may mitigate thisproduce a reliable fabrication.concern to produce larger displacement [8]ElectrostaticIt is simple in the structural system; theThe force is typically small. The greater therequired components are not complex.voltage, the greater the force is, with the limit ofbreak down voltageThe response time to close and opening theThe actuation distance is typically small (<5 μm).valve is generally very small (~ms) if theSmall distance between the membrane anddistance between two surfaces is small (<1 μm).insulation layer increases the clogging possibility.[10, 13]Force generated when the membraneThe actuator dimensions are typically large totouches the insulation layer is extremelyproduce large deflection, this contradict to thelarge; this produces a high sealing force forMEMS application, where a small device isa normally open valve.required.Thermo pneumaticFlexible, the amount of deflection can beThe response time is very long for both openingadjusted based upon the amount of powerand closing the valve. This valve is not suitable forgiven to the heating element. This gives thehigh frequency cycle operation.utilization for a variable valve.Large displacement can be developed in aHeating to the fluid may not be applicable to somesmall valve package [16, 17]application, particularly if the working fluid haslow evaporation temperatureThe implementation of the bistable principleThe vapor pressure change in the fluid inside theis feasible and indicates a significantchamber due to overheating caused by the heaterreduction in the power consumption [16].may cause permanent damage to the membrane.MagneticIt is highly integrated with the actuationThe design may be complex, particularly for thetechnique, such as pre-buckled membranemembrane that involves bi-stable positions.for bi-stable conditions [22] which reducesthe amount of power consumption.It is flexible; the amount of actuation forceThe permanent magnet is usually demagnetizeddepends on the amount of current applied toover the application life.the coil. [3, 4]The time required to open and close isThe magnet saturation of the structure limits thesimilar; it is useful for the application wheremaximum amount of force produced by the coils.reliable and exact time is required.The combination between a permanentmagnet and coils may increase the crushingpressure of valve [3, 4, 5]. This is useful forthe high duty valve, which expose in closinghigh-pressured fluid.HydrogelThere is little power consumption requiredThe volume change of the Hydrogel is diffusion-and suitable for application such as druglimited and exceedingly slow when the path lengthdelivery or other chemicals on demandis large[19, 20]Environmentally sensitive Hydrogel offerSince it relies on the change in environmentalunique opportunities for active flow controlcondition, this valve is not widely applicable for allin micro-flow systems [19]applicationsThe actuation dimension is extremely largeAs it is sensitive to the environmental condition.can more than 100 μm [19]The opening/closing time may not be consistentthrough the operation life
Fluidic connections for the microvalve also present a challenge in the design. In order to simplify the fabrication, most microvalves are fabricated with two or three wafers bonded together, simply to build the inlet and outlet hole [3,4,9,14,17,19-20]. Even though the wafer bonding processes are simple, they are not desirable because it closes the opportunity for the microvalve hybrid integration with the electrical circuit on a single wafer. The yield rate of the wafer bonding is generally low and reduces the fabrication efficiency. Fabricating microvalve from two or three wafers going into a different bath process and finally assemble together, requires a good bonding technique that generally is done at high temperature (above 400° C.), and is clearly not CMOS compatible.
To fabricate a complete set of microvalves fully by surface micro-machining is not easy. The current microvalve art has not yet fully fabricated the whole structure of the valve by MEMS surface micro-machining technique. Some valves were produced by combining parts, which parts are manufactured by conventional machining tool [1-3, 5, 8, 20]. Yet these attempts are not suitable with the spirit of MEMS as a batch fabrication. Thus, this kind of microvalve design will find itself difficult for commercialization in wide industrial area.
In view of the foregoing, it is apparent that it would be beneficial to provide a microvalve fabrication system incorporating the microvalve on a single substrate, preferably a magnetic microvalve. It is to the provision of such a system that the present invention is primarily directed.